The present invention generally relates to the amination of aromatic compounds and to heterocyclic analogs thereof, and specifically, to the direct, catalytic amination of aromatic compounds and heterocyclic analogs thereof. The invention particularly relates, in a preferred embodiment, to the preparation of aniline directly from benzene and ammonia using heterogeneous catalysts.
Current commercial methods for preparing aromatic amines such as aniline involve multiple reaction steps. For example, aniline is typically prepared by converting benzene to a derivative, such as nitrobenzene, phenol or, chlorobenzene, and then converting the derivative to aniline. Such indirect methods, summarized in U.S. Pat. No. 5,861,536 to Durante et al., have long been recognized as less than optimal with respect to corrosive material handling, environmental and/or feedstock cost concerns.
Alternative methods for preparing aromatic amines directly from aromatic hydrocarbons have been reported in the art. For example, aniline can be produced by direct amination of benzene according to Reaction I: 
Reaction I is, however, thermodynamically disfavored in the forward direction at reasonable temperatures and pressures. Approaches have been proposed, therefore, to react the hydrogen produced in Reaction I with oxygen to form water, thereby driving the thermodynamic equilibrium in the forward direction, and improving the conversion of benzene to aniline. The overall reaction according to this approach is represented by Reaction II: 
Canadian patent No. 553,988 to Thomas et al. proposed two distinct embodiments for effecting the approach involving Reaction II. In a first embodiment, Thomas et al. disclose effecting the reaction by contacting benzene, ammonia and gaseous oxygen with a platinum catalyst maintained at a temperature of about 1000° C. Platinum-containing catalysts effective for use in connection with this first embodiment are reported to include, independently, platinum alone, platinum alloyed with certain specifically-recited metals, and platinum combined with certain specifically-recited metal oxides. In a second, independent embodiment, Thomas et al. disclose effecting Reaction II by contacting benzene and ammonia in the vapor phase with a reducible metal oxide at a temperature of from about 100° C. to about 1000° C., without supplying gaseous oxygen to the reaction. The reducible metal oxides said to be suitable for use in connection with this second embodiment include oxides of Fe, Ni, Co, Sn, Sb, Bi and Cu.
Other processes for the direct amination of benzene and other aromatic hydrocarbons have also involved catalysts comprising a reducible metal oxide—with or without also supplying gaseous oxygen to the reactor. U.S. Pat. No. 2,948,755 to Schmerling, for example, describes an approach for effecting Reaction II in which benzene and ammonia, and optionally gaseous oxygen, are reacted in the presence of a catalyst comprising a reducible metal oxide in combination with, independently, molybdenum, tungsten or chromium. U.S. Pat. Nos. 3,919,155 and 3,929,889 to Squire, and U.S. Pat. Nos. 4,001,260 and 4,031,106 to Del Pesco, disclose reacting benzene and ammonia in the presence of a nickel/nickel oxide cataloreactant for effecting Reaction II.
A number ofprocesses for the direct amination of benzene and other aromatic hydrocarbons have also involved catalysts comprising noble metals. Recently, for example, Becker et al. reported the preparation of aniline by reaction of benzene and ammonia with a gaseous oxygen or carbon monoxide co-feed in a plug-flow or continuous-stirred-tank reactor over a Group VIII-metal catalyst. Specific catalysts consisted of, independently, Pd, Pt, Ru, Rh and Ni supported on alumina, and for one experiment, CuO supported on zirconium oxide. (See Becker et al., Amination of Benzene in the Presence of Ammonia Using a Group VIII Metal Supported on a Carrier as a Catalyst, Cat. Let. 54, 124-128 (1998).) The published German patent application DE 19634110 A1 of Hölderich et al. discloses direct amination of benzene with catalysts comprising, independently, Pd, Pt, Rh and Ru. U.S. Pat. No. 5,861,536 to Durante et al. discloses direct oxidative amination of benzene using a supported catalyst comprising a transition metal and a mono- or bi-nucleating ligand. In one example, catalysts comprising palladium with and without a nitroso-group ligand are compared. Axon at al. report, in PCT application WO 99/10311, the reaction of benzene, ammonia and gaseous oxygen in the presence of a catalyst comprising transition metals, lanthanides and actinides, with specific examples involving, independently, a Pt/rh gauze, Pt supported on silica, Pt supported on alumina and V supported on alumina.
The reported approaches for the direct, single-step amination of benzene to aniline have not been adopted commercially. These approaches generally suffer from relatively low benzene conversion and/or relatively low selectivity for aniline. Moreover, the reported catalyst systems involving a metal-oxide cataloreactant are not sufficiently regenerable for commercial viability; that is, such known catalysts do not maintain commercially acceptable benzene conversion and/or aniline selectivity for a commercially-attractive number of cycles. Hence, there remains a need in the art for a commercially viable heterogeneous catalyst and process for the direct amination of benzene and other aromatic compounds or heterocyclic analogs thereof to the corresponding amino compounds.